1. Field of the Invention
The present invention relates to a thermally developable light-sensitive material, and particularly to a thermally developable light-sensitive material which provides pure black tone images and, at the same time, whose shelf life (which means the ability of the thermally developable light-sensitive material to retain those photographic properties initially possessed even after prolonged storage, in other words, "freshness retaining property") is improved, and whose developing speed is further improved.
2. Description of the Prior Art
A variety of methods which comprise subjecting photographic materials containing light-sensitive components such as silver halide or the like to a so-called dry processing by heating to thereby obtain an image are known. Of these light-sensitive materials which can form photographic images using such a dry processing, the most promising one is, at present, a thermally developable light-sensitive material as described in U.S. Pat. Nos. 3,152,904, 3,457,075, 3,707,377 and 3,909,271, etc., in which an oxidation-reduction image forming composition comprising, as essential components, organic silver salt oxidizing agents (for example, silver behenate), photocatalysts such as a small amount of a light-sensitive silver halide and reducing agents is utilized. While the thermally developable light-sensitive material is stable at normal temperature, the organic silver salt oxidizing agent and reducing agent present in the light-sensitive layer undergo, when heated generally at temperatures of higher than about 80.degree. C., preferably greater than 100.degree. C. after exposure to light, an oxidation reduction reaction due to the catalytic action of the photocatalyst such as exposed silver halide which is present in proximity to the organic silver salt oxidizing agent and reducing agent to thereby form silver; whereby the exposed areas of the light-sensitive layer are rapidly darkened so that a contrast is formed between the exposed areas and the unexposed areas (background) to form an image.
In this thermally developable light-sensitive system, the photocatalyst, such as silver halide or the like, remains in the thermally developable light-sensitive material after development without stabilizing the photocatalyst to light. While such can change on exposure to light, the system exhibits the same effect as if stabilized. This is because the photocatalyst such as silver halide or the like is employed in a small amount and the major portions comprise stable white or light colored organic silver salts which are difficultly blackend by light, and therefore, the system appears white or lightly colored as a whole even though a small amount of the photocatalyst such as silver halide or the like changes color upon exposure to light, and such a slight change in color does not degrade the visual appearance.
In most cases, the image provided by the thermally developable light-sensitive material using the above described silver salt oxidizing agent is yellow brown in color. However, it is known that, by incorporating a color toning agent therein, the thermally developable light-sensitive material could be so improved as to obtain an image having a good black tone. U.S. Pat. No. 3,457,075 and the like disclose that phthalazinone can be employed alone as such a color toning agent. Where phthalazinone is used alone, however, disadvantages of a deterioration in shelf life, particularly shelf life at high humidity, are encountered. That is, where the light-sensitive material is exposed to light and developed by heating after storage thereof at high humidity e.g., 80% RH, the color tone of the images obtained becomes brown. The maximum density of the images is also reduced, and furthermore, the sensitivity thereof is also decreased. In addition, phthalazinone very easily sublimes so that, when a number of sheets of thermally developable light-sensitive materials containing phthalazinone are processed with the same processing machine, the processing machine is contaminated by the sublimed phthalazinone. In order to prevent the sublimation of phthalazinone upon heat development, Japanese Patent Application (OPI) No. 67132/75 (which corresponds to U.S. Patent Application Ser. No. 708,450 filed July 26, 1976, now U.S. Pat. No. 4,076,534) discloses phthalazinone derivatives with substituents on the condensed benzene ring thereof. Problems still exist with a thermally developable light-sensitive material using phthalazinone derivatives and the shelf life at high temperature and high humidity needs to be improved, although sublimation is not a problem in the light-sensitive material and the shelf life of the light-sensitive material with the passage of time under normal conditions is excellent. In addition, a thermally developable light-sensitive material comprising a phthalazinone derivative having a carbamoyl group at the N-position thereof as disclosed in U.S. Pat. No. 3,844,797 possesses the disadvantage that the speed of heat development is slow.
On the other hand, examples of compounds heretofore known as a color toning agent, in addition to the above described phthalazinones, include the cyclic imides such as quinalizines, pyrazolin-2-ones and succinic imides as described in U.S. Pat. No. 3,846,136; the benzoxazinediones and quinazolidinediones as described in U.S. Pat. No. 3,951,136; the N-hydroxynaphthalimides as described in U.S. Pat. No. 3,782,941; the mercapto compounds as described in U.S. Pat. Nos. 3,832,186 and 3,881,938; the phthalazinediones as described in Japanese Patent Application No. 116471/73 (which corresponds to U.S. Patent Application Ser. No. 708,450, filed July 26, 1976, now U.S. Pat. No. 4,076,534); the uracils, barbiturates, saccharin, 5-nitrosaccharin, phthalic anhydride, sulfolene, 2-mercaptobenzoxazole, 2-hydroxybenzothiazole, 2-amino-6-methylbenzothiazole, 2-amino-4-(4-biphenylyl)thiazole, imidazole, N,N'-ethylene thiourea, 1-acetyl-2-thiohydantoin as described in British Pat. No. 1,462,016; and the like. However, these known color toning agents have insufficient color toning activity, decrease shelf life or decrease the speed of heat development, which are not desirable.
In addition, the phthalimides as described in British Pat. No. 1,380,795 have the disadvantages of insufficient color toning activity, of delay in speed of heat development, of insufficient shelf life at high humidity, of deterioration of images in color tone, of decrease in maximum density, and of serious decrease in sensitivity, when such a color toning agent is used alone.
Further, U.S. Pat. No. 3,951,136 discloses that toner compounds (for example, unsubstituted phthalazinone, phthalazinone derivatives, phthalimide or phthalimide derivatives) which can ordinarily be employed can be used in combination with benzoxadindiones or quinazolidindiones. However, nothing has been stated regarding the effect of the combination.
Furthermore, U.S. Pat. No. 3,980,482 discloses a thermally developable light-sensitive material which is prepared by adding N,N,N',N'-tetramethylphthalimide as a sensitizing agent prior to halidation of organic silver salts (the procedure whereby a portion of the organic silver salts are halogenated thereby form light sensitive silver halide) and by adding phthalazinone as a color toning agent after the halidation. However, Japanese Patent Application (OPI) No. 7914/76 does not disclose the effect which would be achieved by the combined use of N,N,N',N'-tetramethylphthalimide and phthalazinone. In addition, even if N-methylphthalimide is used in combination with phthalazinone in accordance with the method as described in Japanese Patent Application (OPI) No. 7914/76, the shelf life can not be improved when compared with the use of phthalazinone alone.
As stated above, it is difficult simultaneously to achieve pure black color tone images in a thermally developable light-sensitive material, and to improve the thermally developable light-sensitive material in shelf life at high humidity, and further to improve the thermally developable light-sensitive material in speed of heat development.